The invention relates to a device for a cleaning unit for hydraulic oil or lubricating oil circulating in connection with a drive unit, wherein the cleaning unit comprises a cleaning tank, means arranged for spreading an untreated oil over a substantial portion of the cleaning tank cross-section, an inlet for introducing gas into the untreated oil, and an outlet for evacuating gas and water vapour, and also a means having a large surface area arranged downstream of the spreading means, more particularly by the means having a large surface area being provided as a plurality of bodies arranged to be able to float on the treated oil.
Great demands are made on the oil quality in hydraulic and lubricating systems connected with drive units, engines and the like, particularly where one is dependent on continuous running or where no access to adequate backup solutions in case of an outage of a main plant is available. The oil used must maintain its quality over long periods of time, and from an economical point of view it is desirable to have long replacement intervals for the oil used. To maintain the lubricating and conserving properties of the oil, strict requirements to the oil cleanness are set, i.e. the content of foreign particles and water. It is neither desirable for the oil to contain gases.
Foreign particles may inflict mechanical damage to the moving parts to be lubricated in the form of wear on bearing surfaces and the like.
Water contained in the oil may cause                corrosion;        reduction in the dynamic viscosity, which may lead to weakening or complete loss of the lubricating film; and        changes in the structure of the oil, which may lead to formation of detrimental oxidation products, slime or bacterial growth.        
Air or another gas in the oil may cause                cavitation in the system e.g. in pumps;        local overheating of the oil;        aging/oxidation;        unstable control of hydraulically generated movements; and        weakening of the dynamic lubricating film.        